1. Field of the Invention
The present invention relates to a space optical transmission apparatus and a space optical transmission system for transmitting an optical signal via free space. More particularly, the present invention relates to a space optical transmission apparatus and a space optical transmission system for achieving simultaneous communication with a plurality of terminals.
2. Description of the Background Art
In the field of free-space optical transmission, for example, Japanese Patent Laid-Open Publication No. 2003-309524 (hereinafter referred to as Patent Document 1) discloses a conventional space optical transmission system which achieves communication between a master station and a plurality of terminals. In the conventional space optical transmission system disclosed in Patent Document 1, an optical signal output from a light source provided in the master station is reflected and divided into a plurality of light beams, and angles of the reflected light beams are adjusted so that the light beams are directed toward the respective terminals. FIG. 8 is a diagram illustrating the conventional space optical transmission system disclosed in Patent Document 1.
In FIG. 8, the conventional space optical transmission system is composed of a master station 810 and a plurality of terminals 811a, 811b and 811c. In the master station 810, a light source 812 outputs an optical signal which is modulated using a signal to be transmitted. The optical signal output from the light source 812 is converted into a bundle of light beams substantially parallel to each other by an optical system 813, such as a lens or the like. In the master station 810, mirrors 815a, 815b and 815c for reflecting a portion of the bundle of light beams are provided within the light bundle. Also in the master station 810, angle adjusting mechanisms 814a, 814b and 814c are provided so as to adjust angles of the mirrors 815a, 815b and 815c. For example, the angle of the mirror 814a is adjusted so that a portion reflected by the mirror 81a of the light bundle is emitted toward the terminal 811a. 
Patent Document 1 also discloses a space optical transmission system which employs a plurality of sets of a pair of a space optical transmission apparatus and a terminal. FIG. 9 is a diagram illustrating the conventional space optical transmission system disclosed in Patent Document 1. In the conventional space optical transmission system of FIG. 9, space optical transmission apparatuses 911a, 911b and 911c are provided at a center base 910, respectively corresponding to terminals 921a, 921b and 921c provided in the vicinity of the center base 910. Thus, in the conventional space optical transmission system, by associating the space optical transmission apparatuses 911a, 911b and 911c with the terminals 921a, 921b and 921c, the viewing angle of transmitted light is reduced, so that levels of optical signals received by the terminals 921a, 921b and 921c are prevented from being decreased.
However, in the conventional space optical transmission system of FIG. 8, since an optical signal output from the single light source 812 is reflected and divided, a high-power light source is required, however, there is an upper limit of the power of the optical signal output from the light source 812 for the purposes of a reduction in power consumption and securing of safety of eyes. Therefore, in the conventional space optical transmission system, there is a limitation on the power of optical signals transmitted to the terminals 811a, 811b and 811c, so that high-speed space optical transmission cannot be performed with respect to the terminals 811a, 811b and 811c. 
In addition, in the conventional space optical transmission system of FIG. 9, a large number of space optical transmission apparatuses 911a, 911b and 911c need to be provided in the vicinity of the center base 910, so that the whole system tends to be complicated, and it is difficult to secure a place for providing the space optical transmission apparatuses 911a, 911b and 911c. 